Re: AFR's and timing question.

Does anyone know what is a safe air fuel for my car to be running? I have 99 3.4 in my 986. Also Safe timing? Right now I have the aft's at about 12.8:1 from 4k up and the timing seems to level out about 22Deg. Any info would be great. I have searched elsewhere but nobody had an answer. Thank you.

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Jaay
Does anyone know what is a safe air fuel for my car to be running? I have 99 3.4 in my 986. Also Safe timing? Right now I have the aft's at about 12.8:1 from 4k up and the timing seems to level out about 22Deg. Any info would be great. I have searched elsewhere but nobody had an answer. Thank you.

Nominally the AFR is 14.7:1 (or thereabouts) as a healthy engine being fed this AFR generates exhaust gases which are most efficiently processed by the 3-way converters.

So an AFR of 14.7:1 is safe, under some conditions. But if you monitor AFR under other conditions like for instance hard acceleration it goes down to less than 14.7:1. The engine makes its best power/torque with a richer mixture, I forget the ratio now but call it 12.8:1 which is what you observed.

Timing varies all the (pardon the expression) time. Monitor this in real time and be amazed.

It has been a while since I monitored timing in my cars. The last time I can recall doing this was in my 2006 GTO. (I have done this since in my other cars but I just can't recall the numbers, but they'll be about the same.). Timing ranged from a bit retarded at idle to a few degrees advanced to maybe 35 or 36 degrees advanced under some low RPM high load operation conditions, like cruising in a high gear at very low (~1K) RPMs. It was during these times the instant gas mileage rose to spectacular levels, reaching in some cases 35mpg or a bit more. (This was a 3600+lb car with a 400hp 6.0l V8 engine with a 6--speed transmission.) Unfortunately it was hard to maintain this as anything that made me have to back off the gas pedal then reapply the gas caused the timing to retard and the mpg to drop like a stone.

It is normal for timing to back off under hard acceleration as a richer mixture burns quicker, and doesn't need so much advance.

For Porsches my info is the DME attempts to run as much advance as possible while staying just short of detonation. An early spark gives the fuel mixture time to get burning so that cylinder pressure is peaking at just the best time to convert that energy into mechanical energy.

Provided the engine is stock and has a healthy fuel system and plugs and coils and what have you the AFR should be handled just fine by the DME and there should be no reason to monitor this other than because you apparently have the ability to do so.

Program something never intended for your car?

Diagnose a problem?

Second guess Porsche?

Marc gave a great overview. bear in mind that timing is also affected by the knock sensor whcih retards it if detonation is detected.
Also note that one of the reasons timing varies with RPM is that as the motor spins faster, the burn must occur earlier to effect the same point in the stroke (its needs to catch up to the mechanical motion).

Further - and i'm way out of my depth on this one, air is compressible (and thsu expandable) and can further lag the mechanical action, changing the ideal timing of a spark (assuming we're talking spark timing, not valve timing) - although he same applies to valves.

Anyway, tell us what's up...

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Being that the Maf is not in its stock location, with different plumbing it messes up all the calibration. When I first finished the car it was running rich, down in the 10s. After a little manipulation I was able to get the mixture to where it is today. Anyone that has changed their intakes I would love to see a before and after of their egts and Afrs.

is not so laminar and the MAF can't properly determine the amount of air. This non-laminar air flow does not generally result in a bias one way or the other but causes a chaotic condition with too much fuel in at some ranges of RPMs and too little at others.

You are not the first to attempt to improve the intake system and make it worse. My advice is to if you can put the stock air intake on. Or go to an quality aftermarket air intake system in which someone has spent a lot of time improving the air flow and yet not at the expense of keeping it laminar.

This was not an attempt to make a better intake - though I did and succeeded. I put a 3.4 in the car and needed a custom intake. It is comprised of a 987 airbox, bmw Maf housing and silicone to an ipd plenum. There is a lot more done to the motor too. The car needed to be tuned and is. The only answer I was looking for was the target afr and I got it from a very respected tuner. As for as a quality intake - I've seen just putting on a k&n setup on 996 requires tuning. I don't understand airflow/laminar but I would bet that more owners should get your car on the dyno and get some data from your cars after making modifications and realize the blip in hp is just a result in an over all loss.

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Jaay
This was not an attempt to make a better intake - though I did and succeeded. I put a 3.4 in the car and needed a custom intake. It is comprised of a 987 airbox, bmw Maf housing and silicone to an ipd plenum. There is a lot more done to the motor too. The car needed to be tuned and is. The only answer I was looking for was the target afr and I got it from a very respected tuner. As for as a quality intake - I've seen just putting on a k&n setup on 996 requires tuning. I don't understand airflow/laminar but I would bet that more owners should get your car on the dyno and get some data from your cars after making modifications and realize the blip in hp is just a result in an over all loss.

an early Boxster and the transplant was rather straightforward, although there were several issues to be dealt with. One involved the intake but IIRC the stock 996 intake worked it had to be turned 180 degrees. (Is that right? I wish I could find the copy of the article to confirm...)

Well, I confirmed this by not finding the article I referred to but by visiting Pelican's site and specifically its site on engine swaps. Here's what Pelican has to say on this: "When installing a Carrera 996 engine into the Boxster, you typically use the 996 intake manifold. However, you need to remove it from the engine and then reinstall it later on in the opposite direction. This is so the throttle body will face the rear of the car. I suggest that you take many photos as you remove the manifold, as the routing of the vacuum hoses can be tricky to decipher when you're reassembling the manifold back onto the engine."

Installing a K&N filter should not require any tuning. What often happens is the filter is accompanied by some kind of "cold air intake" feature which simply results in hot air from the engine bay getting into the intake and this upsets the fuel metering. (Also, I'm ignoring the too often encountered problem of an oiled filter fouling the MAF.)

The stock DME and its population of sensors are quite capable of properly fueling an engine even with some modifications. What can happen is the mod's place a higher demand on the MAF and this shortens the MAF's life. At some point many owners of highly modified engines go to a MAF-less system of fuel metering.

The laminar air flow I was referring is important to the MAF's correct function. The MAF consist of a heated film and a control circuit. Incoming air cools this heated film. The MAF's control circuit monitors this and increases the voltage to maintain the specified temperature of the film. The DME monitors the amount of voltage the MAF control circuit uses to keep the hot film at the specified temperature. The voltage signal the DME monitors at the MAF control circuit represents the amount of incoming air.

This system works very very well. But it does have some special requirements. One is the air flow past this film must flow in a laminar manner, without any turbulence. Turbulent air flow results in rapidly varying air flow with a resulting rapidly varying cooling affect and the MAF control circuit can't keep up and the DME can't make sense of the voltage signals and is thus unable to correctly determine the amount of air engine is using at any given time and thus fuel metering is incorrect.

I would go one better than you and recommend any owner contemplating modifications should establish a baseline of not only engine performance (torque/HP) and not only their peak numbers but their curves. Also, the AFR's during all this should be logged. Then after the modifications repeat the measurements to truly know what the gains are and where they are. Often times while there is some gain at peak the curve is changed and not always for the better. And proper AFR's throughout the RPM range are crucial for a long and trouble free engine service life.